Radio receiving system



Dec. 19, 1933. R. s. OHL

RADIO RECEIVING SYSTEM Filed Sept. 17, 1931 lNl/ENTOP R 5 OHL AN E ATTOR/VEY Patented Dec. 19, 1933 UNITED STATES RADIO RECEIVING SYSTEMItussell S. h], Little Silver, N. J., assignor to Bell TelephoneLaboratories,

Incorporated,

New York, N. Y., a corporation of New York Application September 17,1931

Serial No. 563,268

Claims. (Cl. 250 -20) This invention relates to signaling systems andparticularly to radio telephone receiving systems. In radio signalingparticularly at short waves considerable difficulty is sometimesencountered 5 due to fading, that is, variations in the strength of thesignals as received from the transmission medium. Disturbances due tothis cause can often be avoided by the use of an automatic gain controlin the receiver. However, when the fading is such that the strength ofthe received signal falls below the limiting noise level such a methodbecomes inadequate.

An object of this invention is to overcome such fading effects.

It has been observed that fading is a function of frequency and thatoften when its effects are so pronounced as to effectively obliterateone frequency or group of frequencies the transmission at an immediatelyadjacent frequency will be quite satisfactory. In fact it has beenobserved that the sidebands of a modulated wave often fade independentlyof each other.

In a specific preferred embodiment of the present invention the receivedradio wave is modulated by a wave from a local oscillator to produce anintermediate frequency wave which is supplied to two branch circuits,one of which is selective to the wave corresponding to one sideband ofthe receivedwave to the exclusion of the other sideband, and the otheris selective to the frequency corresponding to the other sideband to theexclusion of the first sideband. The outputs of these two selectingcircuits are separately detected so that they may be supprovided inaddition a control path from each selecting circuit. Each of thesecontrol paths contains a rectifier, the output of which is connected toone winding of a differential relay, which is connected so that it willprevent the output of one or the other of the detectors from beingsupplied to the signal circuit. Thus, the signal circuit will at alltimes be responsive to onlyone of the received sidebands and if thefading conditions vary so that the sideband being received at anyinstant fades, the relay will operate to switch the signal circuit tothe other sideband.

The invention may be more fully understood by reference to the followingdetailed description in connection with the drawing which showsdiagrammatically one embodiment thereof.

The radio receiver shown on the drawing is of the double detection type.The received waves are supplied to the high frequency detector 2' pliedto a signal frequency circuit. There is in which they are combined withthe waves from a beating oscillator 3 to produce intermediate frequencywaves which are selected and amplified in the intermediate frequencyamplifier 4.

The output of the amplifier 4 is branched into 3g two separate circuits5 and 6. Circuit 5 contains a high-pass filter 10 designed to select theupper sideband of the intermediate frequency waves,

while the circuit 6 contains a low-pass filter 11 designed to select thelower sideband.

In the circuit 5 the output of the filter 1D is branched into twocircuits, one containing a low frequency detector 11 and the other arectifier 12.

The circuit 6 is similarly branched at the output of filter 20 to a lowfrequency detector 21 to, and a rectifier 22.

In each case the low frequency detectors oper-- ate to combine thecarrier and the respective sidebands to produce the signal wave which isamplified by the audio frequency amplifiers 13 and 23, respectively. Theoutputs of these amplifiers are connected together to the telephonereceiver 7.

The rectifiers 12 and 22 may be of any of the well known types foramplifying and rectifying so the alternating current waves to produce adirect current of instantaneous amplitude determined by the amplitude ofthe respective sidebands supplied thereto. One type of circuit which hasbeen found to be particularly .satisfactory for this purpose is thatdisclosed in my copending application Serial No. 563,265 filed September1'7, 1931. l

The output of the rectifier 12 is connected t one of the sets ofdifferential windings l4 ofoo the polarized relay 9 while the output ofthe rectifier 22 is connected to the other set of differentialwindings24 of the same relay. The armature and contact of the relay 9are connected to the output of the detectors 11 and 21 96 so that whenthe relay is in one position, that is, when operated by the winding 14the output of the detector 21 will be short-circuited and when in theother position, that is, operated by the rectifier by the windings 24the output of MG the detector 11 will be short-circuited. Thus thecircuit will operate so that the telephone receiver 7 will at all timesbe responsive only to the signals derived from one sideband, thedominating sideband operating to short-circuit the branch transmittingthe other sideband.

In order to prevent instability at periods in which the energy level inthe two sidebands is approximately the same or varying rapidly from acondition in which one slightly predominates 10 to a condition in whichthe other slightly predominates, the relay 9 is provided with biasingwindings 30 connected to a battery 31 and potentiometer 32. By properlyadjusting the potentiometer 32 the relay 9 may be adjusted so that therelay is maintained in such position as to short-circuit the circuit 5,for example, under all conditions except when the lower sidebandtransmitted to the circuit 6 dominates the upper sideband by apredetermined amount determined 7 by the bias of the relay. v

The filters 10 and 20 are preferably designed to transmit both therespective sidebands and the carrier suppressing the respective othersideband. However, if desired only the sidebands may be transmitted inwhich case an oscillator 8 is used to supply the carrier to the lowfrequency detectors 11 and 21. Such an oscillator may also be used incases in which no carrier is transmitted from the radio transmitter.

In cases where selective fading is such that there is a markeddifference in fading between the frequencies of a single sideband it maybe desirable to divide each sideband into sub-bands. U. S. Patent1,747,221 to Bcwn, February 18,

1930, discloses a system in which the output of each of a plurality ofreceivers is split into subbands and selection 'made between respectivesub-bands. Each of the two sidebands may be divided in a similar way andthe system of the present invention used for selecting between therespective sub-bands of. the two sidebands.

Such sub-band selectingsystems can be very economically used where asecrecy system of the sub-divided speech band type is employed.

What is claimed is:

1. In a radio receiver for receiving the two sideband componentsresulting from the modulation of a single carrier wave by a signal waveand subjected to varying transmission conditions, means for separatelyreceiving the transmitted sidebands, a signal circuit alternativelyresponsive to the output of the respective sideband receivingmeans, andmeans for automatically rendering said signal circuit responsive to theoutput of one sideband receiving means to the exclusion of the other toselect that sideband which gives an output exceeding a preassignedminimum. v a

2. In a radio receiver for receiving the two sideband componentsresulting from the modulation of a single carrier wave by a signal waveand subjected to varying transmission conditions, means for separatelyreceiving the transmitted sidebands, a signal circuit alternativelyresponsive to the output of the respective sideband receiving means,means for automatically rendering said signal circuit responsive to theoutput of that one of said sideband receiving means the output of whichexceeds that of the other by a preassigned margin, and means forautomatically causing the reselection of the output of the sidebandreceiving means when the energy level of the previously selectedsideband falls below the preassigned margin.

3. In a system for receiving waves representing a single carrier wavemodulated by a signal wave, means for separating the sidebands of thereceived wave, means for demodulating each sideband, and means forautomatically selecting that demodulated, sideband having an energylevel exceeding that of the other by a predetermined margin.

4. In a system for receiving a signal modulated carrier wave comprisingtwo sidebands representing the modulation of a single carrier wave bysignal wave and subjected to varying transmission conditions, a sourceof waves,

means for combining waves from said source with the received. waves toproduce two intermediate frequency sidebands one corresponding to eachof the received sidebands, means for separating said intermediatefrequency sidebands from each other, .means for detecting the separatedsidebands to produce the signal wave from each and means responsive tothe relative energy levels of said sidebands for automatically selectingthe signal output of that one :hich at any instant has an energy levelexceeding that of the other by a pre-assigned margin.

5. In a radio system subject to fading, a radio receiver, means forselecting a frequency band representing-the sum of the transmittedcarrier and-the signal waves,.other means for selecting a frequency bandrepresenting the diiierence between the transmitted carrier and signalwaves, means for detecting each of said frequency bands to. produce thesignal, a signal circuit, a transmission path connecting the output ofeach of the detecting means to said signal circuit, a relay having twowindings for operation in opposite directions, means responsive to theoperation of said relay in one direction for rendering inoperative oneof said transmission paths and responsive to the operation in theopposite direction for rendering inoperative the other of saidtransmission paths, and a rectifier having an input connected to, eachof the selecting means and an output connected to each of said windingsof said relay.

RUSSELL S. OHL.

